EnginSoft Partnership
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Multiphase modeling and fluid dynamics simulation of a biomass gasifier

OBJECTIVE

To use CFD techniques to evaluate some aspects of a prototype biomass gasifier operation, scale 1:4 :

  • Process hydrodynamics evaluation
  • Better understanding of solid circulation
  • Improve equipment simulation
  • Identify possible erosion region in the equipment
  • Obtain results for comparisons with experimental data
METHODOLOGY – GEOMETRY
  • Complete circuit of the gasifier at 1:4 scale
  • Cyclone is substituted by including a porous media
    1. Avoid excessive computational effort to simulate the flow inside the cyclone
  • Simplified loop seal – The simulation represent only the region above the perforated plate (Reduce the computational mesh in the simulation)
    1. 28 holes in feeding chamber
    2. 28 holes in return chamber
METHODOLOGY – PHYSICAL MODEL
  • ANSYS FLUENT Software
  • Turbulence flow regime
    1. k-ε model
  • Multiphase model Eulerian (Ishii, 1975)
  • Interphase momentum transfer
    1. Gidaspow Model (Gidaspow, 1994)
  • Isothermal and incompressible flow
RESULTS – SOLIDS VOLUME FRACTION
  • The methodology is able to promote the solids recirculation throughout the entire gasifier circuit
  • Solids accumulation in stand pipe after 9,75 seconds (start the oscillatory regime)
  • Loop seal promotes the solid transport from stand pipe to riser efficiently
  • The return duct promotes a non-uniform solids distribution in the bottom of the riser
  • Solids volume fraction profiles become more uniform as the flow rises
RESULTS – VELOCITY PROFILE
  • Air velocity higher than solid velocity at riser (upward velocity)
  • Solid velocity higher than air velocity at stand pipe (downward velocity)
  • In the beginning of riser: High velocity in central and left side. Higher positions show more homogeneous distribution
RESULTS – SOLID WALL SHEAR
  • The CFD approach allow to identify possible erosion region in the gasifier
  • Three critical positions
    1. Opposite wall of the return duct in the bottom of the riser
    2. Top of the riser
    3. Loop seal perforated plate (below stand pipe)
  • Qualitative analysis, bum the methodology show possible erosion regions

CONCLUSIONS

Mathematical model is able to reproduce the gasifier operation, presenting consistent results, which the solid recirculation is promoted even without the real cyclone effect.

Identified a solids accumulation in stand pipe and it results in the control of solids flux to riser.

Suspension identified in 2 distinct way in the equipment:

      • High solid concentration and low velocity

      • Low solid concentration and high velocity

Three critical positions with high wall shear stress --> Erosion regions.

Results can be used to get comparisons with experimental data.